hard for me to explain but once you know the difference it will be obvious. aluminum rails tend to be lighter in color and they look thin and light. also, im pretty sure a magnet wont be attracted to aluminum rails so if youre unsure that's a check you could do i suppose haha. also, aluminum won't rust. run something metal over them and it won't slide nearly as well. like the above poster said, it's soft metal compared to your edges and sliding it doesnt work very well at all. let me know if that all helps

in general, just running your ski over the length of the rail with some pressure on it will tell you if there are any trouble spots..that's why you might have seen a lot of cliche shots in movies of the rider doing that